Known membrane carburetors of the above kind include a housing having an intake channel in which a venturi section is formed. An idle nozzle and a main nozzle open into the intake channel. These nozzles supply fuel from a control chamber. The idle output openings lie downstream of the venturi section in the region of the throttle flap. The main nozzle exit opening lies in the region of the venturi section upstream of the throttle flap. In the case of idle, main nozzle drip can occur because of vibrations as well as because of the pulsating gas mixture in the intake channel. This can lead to an enrichment of the mixture and the idle fluctuations associated therewith. For this reason, a check valve is provided in the fuel tank of the main nozzle at the end next to the intake channel. This check valve comprises a valve platelet, which is provided in a valve chamber, and a valve seat assigned thereto. The valve platelet mechanically closes the main nozzle path in the case of idle so that pressure pulsations in the intake channel do not operate on the control chamber via the main nozzle path. Under unfavorable conditions, main nozzle drip occurs nonetheless in individual cases which can lead to an overenrichment of the mixture and a stalling of the engine.